I. Field of the Invention
The present invention relates to communication systems, and, more particularly, to a novel and improved method and apparatus for communicating information in a spread spectrum communication system.
II. Description of the Related Art
Communication systems have been developed to allow transmission of information signals from a source location to a physically distinct user destination. Both analog and digital methods have been used to transmit such information signals over communication channels linking the source and user locations. Digital methods tend to afford several advantages relative to analog techniques, including, for example, improved immunity to channel noise and interference, increased capacity, and improved security of communication.
In transmitting an information signal from a source location over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave on the basis of the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the channel bandwidth. At the signal reception location the original message signal is reproduced from the received modulated signal. Such reproduction is generally achieved by using an inverse of the modulation process employed by the source transmitter.
Modulation also facilitates multiple-access, i.e., the simultaneous transmission of several signals over a common channel. Multiple-access communication systems will often include a plurality of remote subscriber units requiring intermittent service of relatively short duration rather than continuous access to the communication channel. Systems designed to enable communication over brief periods of time with a set of subscriber units have been termed multiple access communication systems.
A particular type of multiple access communication system is known as a spread spectrum system. In spread spectrum systems, the modulation technique utilized results in a spreading of the transmitted signal over a frequency band wider than the bandwidth of the signal being transmitted. One type of multiple access spread spectrum system is a code division multiple access (CDMA) modulation system. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention.
In the above-referenced U.S. Pat. No. 4,901,307, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
More particularly, communication in a CDMA system between a pair of locations is achieved by spreading each transmitted signal over the channel bandwidth by using a unique user spreading code. Specific transmitted signals are extracted from the communication channel by despreading the composite signal energy in the communication channel with the user spreading code associated with the transmitted signal to be extracted.
An improved method for communicating information signals in a spread spectrum communication system was disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", which is also assigned to the assignee of the present invention, and which is herein incorporated by reference. The CDMA system as disclosed in U.S. Pat. No. 5,103,459 (the '459 patent) contemplated spreading all signals transmitted by a cell or one of the sectors of the cell with an "outer" pseudonoise (PN) code for both the in-phase (I) and quadrature-phase (Q) channels. The signals were also spread with an inner orthogonal code generated by using Walsh functions. A signal addressed to a particular user was multiplied by the outer PN sequences and by a particular Walsh sequence, or sequence of Walsh sequences, assigned by the system controller for the duration of the user's telephone call. The same inner code was applied to both the I and Q channels resulting in a modulation which is effectively bi-phase for the inner code. Constructing PN sequences which are orthogonal reduces mutual interference between users, allowing higher capacity and better link performance.
It is well known in the art that a set of n orthogonal binary sequences, each of length n, for n any power of 2 can be constructed, see Digital Communications with Space Applications, S. W. Golomb et al., Prentice-Hall, Inc, 1964, pp. 45-64. In fact, orthogonal binary sequence sets are also known for most lengths which are multiples of four and less than two hundred. One class of such sequences that is easy to generate is called the Walsh functions.
A Hadamard matrix of order n can be defined recursively as follows: ##EQU1## where H' denotes the logical complement of H, and H(1)=0. Thus, ##EQU2## H(8) is as follows: ##EQU3## A Walsh sequence is one of the rows of a Hadamard matrix. A Hadamard matrix of order n contains n sequences, each of length n bits.
A Walsh function of order n (as well as other orthogonal functions) has the property that the cross-correlation between all the different sequences within the set is zero, provided that the sequences are time aligned with each other. This can be seen by noting that every sequence differs from every other sequence in exactly half of its bits. It should also be noted that there is always one sequence containing all zeroes and that all the other sequences contain half ones and half zeroes.
Neighboring cells and sectors can reuse the Walsh sequences because the outer PN codes used in neighboring cells and sectors are distinct. Because of the differing propagation times for signals between a particular mobile's location and two or more different cells, it is not possible to satisfy the condition of time alignment required for Walsh function orthogonality for both cells at all times. Thus, reliance must be placed on the outer PN code to provide discrimination between signals arriving at the mobile unit from different cells. However, all the signals transmitted by a cell are orthogonal to each other and thus do not contribute interference to each other. This eliminates the majority of the interference in most locations, allowing a higher capacity to be obtained.
In the system of the '459 patent Walsh functions were also employed to encode the channel data signals transmitted over both the cell-to-mobile link (i.e., the "forward" link) and the mobile-to-cell link (i.e., the "reverse" link). In the exemplary forward link numerology as disclosed therein, a total of 64 different Walsh sequences were available with three of these sequences dedicated to the pilot, sync and paging channel functions. In the sync, paging and voice channels, input data was convolutionally encoded and then interleaved as is well known in the art. Furthermore, the convolutional encoded data was also provided with repetition before interleaving as is also well known in the art.
A similar 64-ary orthogonal signaling technique using Walsh functions is described with reference to the reverse link of the system of the '459 patent. The message encoding and modulation process on the reverse link begins with a convolutional encoder of constraint length K=9 and code rate r=1/3. At a nominal data rate of 9600 bits per second, the encoder produces 28800 binary symbols per second. These are grouped into characters containing 6 symbols each at a rate of 4800 characters per second with there being 64 possible characters. Each character is encoded into a length 64 Walsh sequence containing 64 binary bits or "chips."
The encoding method described with reference to the reverse link is, however, less than optimal in that certain information is redundantly carried by each 64 chip Walsh sequence. It is therefore an object of the invention to provide a Walsh encoding technique which improves information carrying capacity by reducing such redundant information transmission.